1. Field of the Invention
The present invention relates to a solid-state imaging device in which a light-receiving plane has an embedded photoelectric conversion area.
2. Description of the Related Art
As a result of recent proliferation of digital cameras, camera-equipped portable cellular phones, and the like, demand for a solid-state imaging device has increased. In particular, there is increasing demand for a CMOS solid-state imaging device capable of being manufactured through CMOS processes which are common semiconductor manufacturing processes. In relation to such solid-state imaging devices, further increasing demand exists for further miniaturization and an increase in the number of pixels, and miniaturization of a pixel size has posed an important problem.
However, the amount of light incident on the solid-state imaging device is also decreased in conjunction with miniaturization of the pixel size, which in turn decreases signal charges of the incident light. There is a fear of deterioration of detection sensitivity, a decrease in an signal-to-noise ratio, and the like, which would otherwise be cause by the decrease in signal charges.
In the meantime, the area of a photodiode in the solid-state imaging device is also reduced along with miniaturization of the pixel size, which in turn raises another problem of a decrease in the number of saturated electrons of the photodiode that takes control of the upper limit of a dynamic range.
Since the number of saturated electrons in the photodiode is determined by the number of dopants (the concentration of a dopant) in an n-type dopant region forming the photodiode, the number of saturated electrons can be in principle increased by increasing the concentration of dopant.
Moreover, in order to reduce noise which takes control of the lower limit of the dynamic range, an ordinary image sensor usually uses an embedded photodiode, in which a heavily-doped p-type dopant diffusion layer is formed as a photodiode structure in the vicinity of the surface of a semiconductor, to thus shield the surface. The photodiode of such a structure yields an effect of reducing noise by virtue of the surface shield, as well as exhibiting a considerably-superior characteristic of the ability to transfer electric charges from the photodiode to a charge detection section in a perfect transfer mode by making a design so that a completely depletion occurs when the potential of the photodiode is lower than a channel potential achieved at the time of activation of the transfer transistor, and the ability to completely eliminate a residual image or reset noise, which would otherwise arise when transfer of electric charges from the photodiode is incomplete.
However, increasing the concentration of dopant of the n-type dopant region forming the photodiode with a view toward increasing the number of saturated electrons signifies an increase in the potential for depleting the photodiode, which in turn poses difficulty in realization of perfect transfer. The amount of dopant and a dopant profile—which have been optimized by conditions for perfect transfer—are optimized by means of a channel potential achieved at the time of activation of a transfer transistor, the potential of the depleted photodiode, the depth of the maximum potential section, and the like. Consequently, the number of saturated electrons determined by means of the number of n-type dopants is determined by the surface area of the photodiode, and increasing the surface area of the photodiode cannot be performed.
For the purpose of preventing deterioration of a sensitivity characteristic, which would otherwise be caused in conjunction with a decrease in light-receiving area, and to enhance the charge conversion efficiency of incident light, there is disclosed a technique for forming a V-shaped trench in a light-receiving plane of a light-receiving section of a solid-state imaging device (see; e.g., JP-A-6-5827.)
However, the technique mentioned in JP-A-6-5827 is described in connection with enhancement of conversion efficiency which is achieved by means of repeatedly causing the light incident on the V trench to undergo reflection in the V trench, to thus re-enter the V trench. However, no description is provided in connection with an increase in the number of saturated electrodes of the photodiode which is achieved while a complete transfer mode is maintained, nor is a suggestion thereon described.
Further, in relation to all pixels of a single-panel color imaging device, no consideration is given to deterioration of a device characteristic, which would otherwise be caused by forming a photodiode having a V-shaped trench.